Summary

Fixed nitrogen accumulated by legumes is the main nitrogen source for organic farming systems. Knowledge about the amount of fixed nitrogen, its pathways into forage yield, crop residues, soil-N and yield formation of the following crop is needed for designing crop rotations. In a field experiment conducted in Northern Germany differently managed (cut, mulched) legumes (red clover, alfalfa, white clover) in pure stands and in mixture with a companion grass (Lolium perenne) have been grown to determine Yield, forage-quality, N2-fixation and residue nitrogen. Cropped grass/legume reached higher N2-fixation than mulched. While green manure grass/legume left up to 296 kg ha-1 of N in mulch, stubble and roots on the field, most cropped grass/legume mixtures left less than 120 kg N ha-1 in crop residues. Cropped swards showed higher N2-fixation than the mulched mixtures. Swards with red clover or alfalfa reached higher legume contents and harvestable biomass than swards with white clover. Conclusions The results show that biomass production, forage yields, residue nitrogen and N fixation of grass/legume mixtures can be influenced by various combinations of legume species and management. The factors listed have to be considered carefully when planning crop rotations.